1. Field of the Invention
The present invention relates to a developing agent for developing an electrostatic charge image or a magnetic latent image in an electrophotographic method, an electrostatic printing method, a magnetic recording method and the like and in particular, to an encapsulated developing agent.
2. Description of the Related Art
In the related-art production method of an electrostatic charge image developing toner, a kneading pulverization method was the main current. In the case of a toner particle to be produced by the kneading pulverization method, in general, its shape is amorphous, and its surface composition is heterogeneous. Though the shape or surface composition of the toner particle slightly changes depending upon pulverization properties of a material to be used or conditions of a pulverization step, it was difficult to intentionally control the shape. Also, the kneading pulverization method was limited in minimizing the particle size. Mechanical pulverization of a toner goes through steps of pulverization, classification and the like. When the particle size becomes small, the yield is lowered due to a reduction in the efficiency in the classification, and necessary energy increases. On the other hand, with the diffusion of a digital color system aiming at a higher image quality of these days, needs for realizing a small particle size of the toner have been increased. The small-particle sized toner is able to increase a coverage on a medium such as paper at a low consumption amount and is especially advantageous for colorization of electrophotography. Also, from the viewpoints of transfer properties and fixability, it is demanded to precisely control a toner particle regarding the toner shape, particle size distribution and encapsulation and the like. As a production method which meets these demands, the development of a toner by a polymerization method has been advanced in recent years.
Also, in recent years, in view of the matter that an image with higher image quality due to coloration is demanded, investigations of a polyester based resin which is able to be fixed at lower temperatures as a kind of a toner binder resin are being advanced. Furthermore, following the development of a polymerized toner, it is also actually investigated to encapsulate a core agent with a shell material. For example, as disclosed in Japanese Patent No. 3141783, there is proposed a production method by encapsulation in a dispersion. By performing the encapsulation, it becomes possible to achieve low-temperature fixing such that a releasing agent is not exposed on the toner surface.
In a developing agent, in general, a pigment and a releasing agent component are contained together with a binder resin. The releasing agent component is added for the purpose of widening an offset region of high temperature and low temperature in a fixing unit. In general, when the amount of the releasing agent component increases, releasing properties against a heat fusing roll are improved, and a non-offset region is widened, thereby contributing to electric power saving in a fixing step. But, especially in a state that the releasing agent component is exposed on the toner surface, a problem of filming on a control blade of a development unit or a surface of a photoreceptor is generated. Also, the preservability of the toner at high temperatures becomes worse, and a blocking phenomenon occurs. Then, an electrostatic charge image developing toner having an encapsulated structure in which a releasing agent is dispersed in a core resin, and a releasing agent component does not exist in a shell resin provided on the surface of the core resin has been demanded.
But, in the case where a toner is produced through dispersion and coagulation steps in an aqueous solution by forming a core agent by using a polyester based resin, the dispersibility of a pigment or a releasing agent in the core resin is a problem. When compatibility of the pigment or releasing agent with a so-called binder resin is poor, there is a possibility that the pigment or releasing agent is separated during the foregoing dispersion and coagulation steps. With respect to this point of issue, the case where the dispersion of the releasing agent is insufficient was seen even in the kneading step by the related-art pulverization method. However, in the production method of a polymerized toner by treatment in an aqueous solution, there may be the case where the pigment or releasing agent dispersion and the resin dispersion are completely separated, and severer compatibility is demanded. For example, as disclosed in Japanese Patent No. 3716847, a technology in which a toner has a core/shell structure, both a core resin and a shell resin are a polyester based resin, and a releasing agent is composed of an ester compound is reported. But, this technology involves defects that it is of a three-layered structure and is complicated in a process and that since the shell resin is a polyester based resin, in general, it is easily affected by an environmental fluctuation of the charge quantity. Also, for example, as disclosed in JP-A-2006-84952, a technology of encapsulating a polyester based core resin with a styrene-acrylic shell resin is reported. But, no particular description regarding a releasing agent is given, and there was a risk that the releasing agent is separated.
Also, as to the releasing agent, paraffin based materials or olefin based materials were frequently used. But, from the viewpoint of dispersion of a releasing agent in a polyester based resin, a releasing agent which is richer in compatibility has been desired.
In order to solve the foregoing problems, the invention is to provide a developing agent having excellent low-temperature fixability and satisfactory preservability and charge stability.